Lateral ligand-receptor interactions on membranes probed by simultaneous fluorescence-interference detection

Understanding how receptors assemble in membranes remains a challenge in cell biology. Prior research has shown that receptors can cluster in response to ligand binding. However, the exact dynamics of lateral interactions on membranes are not fully resolved. Established methods often lack the ability to track real-time interactions in fluid membranes. This gap motivated the development of new approaches to study receptor-ligand interactions in situ. The need for simultaneous detection of both fluorescence and interferometric signals became apparent. Techniques like total internal reflection fluorescence spectroscopy have been used before. Yet, their application to membrane-bound receptor dynamics is limited. This paper introduces a novel method to probe lateral ligand-receptor interactions in membranes.

The goal of this work is to develop a method for observing ligand-receptor interactions in membrane environments. The focus is on the type I interferon receptor subunits, ifnar1-EC and ifnar2-EC. The aim is to mimic natural membrane conditions by anchoring receptors onto fluid lipid bilayers. The researchers seek to track how ligands induce receptor clustering. The motivation stems from the need to understand the sequential steps in receptor assembly. The study also aims to quantify interaction rates and equilibrium constants. By using labeled IFNalpha2, the team can monitor ligand binding in real time. This approach allows for precise control of receptor surface concentrations and binding dynamics.

The extracellular domains of ifnar1-EC and ifnar2-EC were immobilized on fluid lipid bilayers. These bilayers were supported on solid substrates to maintain membrane-like conditions. Fluorescence-labeled IFNalpha2 was used to track ligand binding to the receptors. Simultaneous total internal reflection fluorescence spectroscopy and reflectance interferometry were employed. These tools allowed for real-time monitoring of ligand-receptor interactions. The researchers characterized the binding of IFNalpha2 to each receptor subunit separately. By varying receptor surface concentrations, they determined equilibrium and rate constants. Mutants of IFNalpha2 with altered association rates were also tested to assess their effect on assembly.

The data suggest that IFNalpha2 binds first to ifnar2-EC, followed by recruitment of ifnar1-EC. This two-step process was observed using simultaneous fluorescence and interferometric detection. The ternary complex formation was confirmed through ligand dissociation measurements. Receptor surface concentrations were controlled to determine binding constants. Mutants of IFNalpha2 showed different association rates with ifnar2-EC. These differences affected the overall receptor assembly dynamics. The study found that the rate of ligand binding to ifnar2-EC is critical for complex formation. Interferometry provided precise quantification of membrane interactions. Fluorescence spectroscopy revealed spatial and temporal details of receptor clustering.

The authors propose that ligand-induced receptor assembly occurs in two distinct steps. The first step involves ligand binding to ifnar2-EC, followed by recruitment of ifnar1-EC. The study supports the role of association rate constants in determining assembly efficiency. The use of simultaneous fluorescence and interferometric detection proved effective. The method allows for precise control of receptor surface concentrations. The findings suggest that membrane dynamics play a key role in receptor clustering. The researchers emphasize the importance of lateral diffusion in membrane-bound interactions. These results may inform future studies on receptor-ligand interactions in biological membranes.